In recent years, electro-optic devices using ferroelectric liquid crystals have attracted increasing attention. An electro-optic device using a ferroelectric liquid crystal is characterized by its extremely highspeed response, bistability and the like, as compared with a conventional electro-optic device using a nematic liquid crystal. As an electro-optic device capable of exhibiting such characteristics associated with a ferroelectric liquid crystal, a surface stabilized ferroelectric liquid crystal (hereinafter referred to as SSFLC) cell is available. This cell was developed by Clark et al. (Japanese Laid-Open Patent Publication No. 107216/81), and is said to be a cell which is most expected to be put to practical use.
A SSFLC cell is characterized in that when a ferroelectric liquid crystal is retained in a cell (haying a cell gap of about 0.5-2 .mu.m) which is so thin as to enable to a helical structure thereof to disappear, bistable states of liquid crystal molecules manifest themselves. In other words, under two stable states, the liquid crystal molecules are in uniform states which differ in the direction of a director being a unit vector in the direction of a long axis of the liquid crystal molecule. Since the ferroelectric liquid crystal has spontaneous polarization and bistability as mentioned above, high-speed response and memory characteristics can be obtained. Therefore, its application to a high-density large-screen display and a high-speed light shutter, etc., has been studied. And great progress has been made in the study of SSFLC cells utilizing said bistable states, with the recent development of liquid crystal materials.
However, there remain some problems with the SSFLC cells which have yet to be solved. For example, there are problems related to orientation reliability and sticking. Once an SSFLC cell is thrown into orientation disorder by a physical shock, etc., it is impossible to readily make an orientation recovery, unlike a nematic liquid crystal cell. Therefore, a great deal of studies have been made of methods for producing a display panel having resistance to any physical shock, but they have not yet succeeded in obtaining such reliability as in the case of a nematic liquid crystal. Furthermore, sticking is a phenomenon in which an image written on a liquid crystal display remains for an indefinite time, and is said to be a ghost-effect. The cause of sticking lies in the spontaneous polarization of a ferroelectric liquid crystal. In other words, sticking is a problem that cannot be avoided as long as a ferroelectric liquid crystal is used. Researches for avoiding sticking have also been made extensively, but a solution to the problem has not been found as yet.
Moreover, researches have been made on a liquid crystal cell (antiferroelectric liquid crystal cell) utilizing a liquid crystal having an antiferroelectric phase (hereinafter referred to as an antiferroelectric liquid crystal) recently discovered as a novel smectic phase (Japanese Journal of Applied Physics, Vol. 27, pp. L729-L732, 1988), which is different from said ferroelectric liquid crystal. This cell is characterized by its capability of performing tristable switching, its distinct threshold characteristic, its good memory characteristic and the like. FIG. 1 shows the relationship between applied voltage and tilt angle in the case of an antiferroelectric liquid crystal cell. It can be perceived from this figure that there are three stable states in an antiferroelectric liquid crystal cell. That is, they are two uniform states (Ur, Ul) as observed in ferroelectric liquid crystal cells, and the third state. It has been reported by Chandani et al. that this third state is a manifestation of the antiferroelectric phase (Japanese Journal of Applied Physics, Vol. 28, pp. L1261-L1264, 1989).
Thus, in an antiferroelectric liquid crystal cell, switching occurs between three stable states. Furthermore, in FIG. 1, for example, in case positive voltage is gradually applied, a change in tilt angle occurs in the sequence of A-B-C-D. The tilt angle hardly changes when the applied voltage is between 0(V) and V.sub.1 (V), but greatly changes when the applied voltage exceeds V.sub.1 (V). Next, in case the voltage is gradually lowered, the tilt angle changes in the sequence of D-E-F-A. In this case, it greatly changes when the applied voltage becomes less than V.sub.2 (V). The same applies to the case of negative voltage application. Thus, the antiferroelectric liquid crystal cell has distinct thresholds with respect to applied voltage. Moreover, from the fact that threshold V.sub.1 (V) in the voltage-raising step is different from threshold V.sub.2 (V) in the voltage-lowering step, it can be perceived that the antiferroelectric liquid crystal cell has a memory characteristic. In fact, for driving a cell utilizing a memory characteristic, it is necessary to apply pulsed voltage to the cell, in addition to bias voltage, as suggested by Chandani et al (Japanese Journal of Applied Physics, Vol. 27, pp. L729-L732, 1988). In this case, it is possible to perform switching within the memory characteristic between the third state and a uniform state.
The most important reason why attention has currently been riveted upon antiferroelectric liquid crystals is that they are expected to be used as liquid crystals with which the various problems of ferroelectric liquid crystals can be all resolved. For example, an antiferroelectric liquid crystal has a self-recovery function for orientation, whereby orientation reliability is greatly enhanced, as compared with a ferroelectric liquid crystal. Also, there is no permanent spontaneous polarization in the antiferroelectric liquid crystal, and therefore, the phenomenon of sticking can be avoided. However, there still exist many problems which remain to be solved regarding the antiferroelectric liquid crystal. Such problems include those related to response time and tilt angle. In the case of the antiferroelectric liquid crystal, it is possible to perform three types of switching. That is, they are "third state.fwdarw.uniform state", "uniform state.fwdarw.third state" and "uniform state.fwdarw.uniform state" switchings. For drive by means of utilizing a memory characteristic, it is necessary to perform switching between the third state and a uniform state, as mentioned above. But, as investigated in Japanese Journal of Applied Physics, Vol. 30, pp. 2380-2383, 1991, the response time generally decreases in the following order: "uniform state.fwdarw.third state" .fwdarw."third state .fwdarw.uniform state".fwdarw."uniform state.fwdarw.uniform state".
Furthermore, in general, the response time for the "uniform state.fwdarw.third state" transition is about 10 times to 100 times as much as the response time for the "third state.fwdarw.uniform state" transition. As for switching utilizing a memory characteristic, two types of switching with response times having large values are used. In light of the response time alone, it is desirable to utilize the "uniform state.fwdarw.uniform state" switching, but it has been considered to be unsuitable for a simple-matrix display because there is no memory characteristic.
In addition to response time, there is a problem with tilt angle. In view of memory drive suggested by Chandani et al., the optimum tilt angle is 45 degrees. However, many of the tilt angles of antiferroelectric liquid crystals hitherto synthesized are 35 degrees or less, and it is difficult to sufficiently increase the optical transmittance of a cell. If it is possible to utilize the "uniform state.fwdarw.uniform state" switching, the optimum tilt angle becomes 22.5 degrees, and sufficient optical transmittances can be obtained even with the antiferroelectric liquid crystals hitherto synthesized.